Pronounced effects of Ni(P) thickness on the interfacial reaction and high impact resistance of the solder/Au/Pd(P)/Ni(P)/Cu reactive system

Abstract The effects of Ni(P) thickness, δ Ni(P) , on the interfacial reaction and high impact resistance of Sn-3Ag-0.5Cu/Au/Pd(P)/Ni(P)/Cu microelectronic solder joints were investigated. During soldering, the surface layers of Au (0.1 μm) and Pd(P) (0.2 μm) were readily eliminated from the interface, and the Ni(P)/Cu structure underneath subsequently came into contact with the solder. When δ Ni(P) was 0 μm, a solder/Cu reaction occurred, and the intermetallic compounds (IMCs) Cu 3 Sn and Cu 6 Sn 5 (dissolved with 1–3 at.% Pd) formed at the interface. When δ Ni(P) was sufficiently thick (i.e., 7 μm), the reaction between the components transformed the system into a solder/Ni(P) system with multilayer IMCs, which included (Cu,Ni) 6 Sn 5 , (Ni,Cu) 3 Sn 4 , Ni 2 SnP, and Ni 3 P. On the submicron scale (e.g., δ Ni(P)  = 0.9 μm), the solder/Ni(P) and solder/Cu reactions can occur sequentially, indicating that a submicron-thick Ni(P) layer did not function as an efficient diffusion barrier between solder and Cu. With the aid of Cu–Ni–Sn phase diagram, diffusion path predictions can be made about the IMC translation in response of different δ Ni(P) . High-speed ball shear testing showed that the impact resistance of the solder joints was significantly reduced with increasing δ Ni(P) . These findings suggested that δ Ni(P) is an important factor in the interfacial microstructure and the resulting mechanical properties of solder joints and that the direct deposition of an Au/Pd(P) dual layer (i.e., δ Ni(P)  = 0 μm) on top of the Cu pads can offer an appropriate solderability for the lead-free Sn–Ag–Cu alloy.